There is nothing quite as satisfying as slicing into a perfectly cooked, edge-to-edge medium-rare steak. If you are looking for a foolproof beef tenderloin steak recipe (that works just as well with Trader Joe’s Filet Mignon), this method guarantees melt-in-your-mouth tenderness every single time—without expensive kitchen gadgets or stressful skillet timing.
The secret lies in combining two gentle culinary techniques: enzymatic seasoning with Japanese Koji and an effortless rice cooker “sous-vide” water bath. By utilizing the deep umami of Shoyu Koji (soy sauce fermented with rice koji) and Ama Koji (sweet rice koji paste), the meat marinates in pure natural flavor while low-temperature cooking keeps it extraordinarily juicy.
Filet Mignon vs. Beef Tenderloin Steak: Which Should You Use?
Both cuts work flawlessly with this exact recipe:
- Filet Mignon is cut from the smaller, tapered end of the tenderloin muscle. It is prized for its buttery, delicate texture.
- Beef Tenderloin Steaks come from the central portion of the same muscle. They share the exact same tender grain structure and lean profile.
Because both are lean cuts, traditional high-heat roasting can easily dry them out. Cooking them in a gentle 140°F (60°C) water bath preserves every drop of moisture while the koji enzymes lock in maximum flavor.
The Culinary Magic of Shoyu Koji & Ama Koji
Instead of relying on heavy sugar or harsh acidic marinades, this sauce harnesses natural Japanese fermentation:
- Shoyu Koji: Rich in protease enzymes, it breaks down surface proteins into savory amino acids (umami).
- Ama Koji (Amazake Paste): Adds a subtle, natural sweetness that balances the soy sauce and garlic without masking the natural beef flavor.
- Golden Sear: A fast, high-heat sear upfront creates a fragrant golden-brown crust (kitsune-iro), layering roasted notes on top of the deep savory sauce.
Ingredients for This Beef Tenderloin Steak Recipe
To pull off this tender, flavor-packed roast beef, you only need a handful of quality ingredients:
- The Cut: 2 thick-cut Beef Tenderloin Steaks (or Filet Mignon), approximately 0.6 lbs each.
- The Precision Seasoning: Fine sea salt measured accurately to 1% of the meat’s weight (8g).
- The Umami Marinade Base: A balanced blend of Shoyu Koji (soy sauce koji) and Ama Koji (sweet rice koji paste), paired with minced onion, garlic, mirin, and red wine.
- The Pan-Sear & Sauce Finish: High-heat avocado oil to form a golden crust, a splash of apple cider vinegar to brighten the reduction, and an optional knob of butter for a glossy finish.
Step-by-Step Method at a Glance
- Temper & Season: Salt the steaks and let them sit at room temperature for 1–2 hours.
- Sear: Quickly sear all sides in a hot skillet for 60–90 seconds for a rich crust.
- Bag & Marinate: Mix the koji marinade directly in a Ziploc bag, add the seared beef, coat evenly, and press out all air.
- Gentle Water Bath: Submerge in a 140°F (60°C) water bath in your rice cooker on “Keep Warm” for 45 to 60 minutes.
- Reduce & Serve: Rest the meat, reduce the bag juices into a velvety sauce in a skillet, slice, and drizzle!
Beef Tenderloin Steak (or Filet Mignon) with Savory Koji Sauce
Equipment
- 1 Rice Cooker / Sous-Vide
Ingredients
The Beef & Tempering
- 2 pieces Beef Tenderloin Steak or Filet Mignon (approx. 0.6 lbs each, 1.2 to 1.4 lbs total)
- 8 g Sea Salt (accurately scaled to 1% of the total meat weight)
- 1-2 tbsp Avocado Oil (or any high-smoke-point oil for searing)
The Savory Koji Reduction Sauce
- 1/2 Medium Onion (finely minced)
- 2.5 tbsp Shoyu Koji (Soy Sauce Koji)
- 2.5 tbsp Ama Koji (Sweet Rice Koji / Amazake paste)
- 2 tbsp Hon Mirin
- 2 tsp Grated Garlic
- 1 tbsp Red Wine
- 1/2 tbsp Apple Cider Vinegar (or Balsamic Vinegar, reserved for reducing)
- 1 tbsp Unsalted Butter (optional, for finishing)
Instructions
Temper and Season the Meat
- Remove the beef from the refrigerator and place it on a clean tray. Evenly sprinkle the 8g of sea salt over all sides of the beef, gently rubbing it in. Let the meat sit at room temperature for 1 to 2 hours until it no longer feels cold to the touch.
High-Heat Golden Sear
- Heat a skillet over medium-high heat and add 1 to 2 tablespoons of avocado oil. Sear the beef for 60 to 90 seconds per side until a deep golden-brown crust forms. Sear the narrower sides for about 40 seconds.
- Remove the beef from the pan and let it cool slightly.
Prepare Sauce Marinade & Seal
- In a heavy-duty, heat-safe resealable bag (such as a Ziploc freezer bag), add the minced onion, shoyu koji, ama koji, hon mirin, grated garlic, and red wine. Mix well with a small flexible spatula directly inside the bag until evenly combined.
- Add the slightly cooled beef to the bag. Turn and massage the meat through the bag (or use your spatula) to ensure every part of the steak is evenly coated in the koji sauce.
- Carefully press out as much air as possible to create a tight vacuum seal, then seal tightly. (Double-bag if needed).
Gentle Rice Cooker "Sous-Vide"
- Fill your rice cooker inner pot with warm water heated to 140°F (60°C).Place a small heat-safe bowl or silicone mat upside down at the bottom of the pot to prevent the bag from resting directly on the heat source.
- Submerge the bagged beef completely. (If the bag floats, place a small heat-safe saucer on top as a weight).
- Close the lid, set to “Keep Warm”, and hold for 45 minutes (extend to 1 hour if cuts are very thick).
- Remove the bag from the water bath. Take the beef out of the bag, place it on a plate, and tent loosely with aluminum foil for 5 to 10 minutes to retain its juices. Do not discard the marinade or juices in the bag.
Reduce the Koji Sauce
- Pour all the remaining liquid and minced onions from the bag into your skillet.
- Add 1/2 tablespoon of apple cider vinegar (or balsamic).Simmer over medium heat until the sauce reduces into a glossy, slightly thickened glaze.
- Optional: Whisk in 1 tablespoon of cold butter right at the end for extra richness. Taste and adjust with pepper if desired.
Slice and Serve
- Slice the beef into clean medallions against the grain.
- Arrange on a serving platter and spoon the warm shoyu koji reduction generously over the top.
Notes
- Meal Prep Friendly: This roast beef keeps exceptionally well in the refrigerator. Cook it in advance, keep it wrapped whole, and slice cold for bento boxes, grain bowls, or crisp salads!
- Equipment Note: If you own a dedicated Sous-Vide precision cooker or yogurt maker, set it to 140°F (60°C) for identical results.
Nutrition
FAQ: About This Beef Tenderloin Steak Recipe
Q: What if I don’t have a rice cooker or a sous-vide machine?
A: You can easily replicate this gentle water bath using a heavy-bottomed pot (like a Dutch oven) on your stove. Bring water to 140°F–145°F (60°C–63°C), place a small heat-safe plate at the bottom so the bag doesn’t touch direct heat, turn the heat to the lowest setting, cover, and submerge your sealed bag for 45 to 60 minutes.
Q: How do you know if the meat is cooked properly on the inside?
A: Because we hold the water bath at a steady 140°F (60°C), the internal temperature of the beef gradually equalizes to match the water. This guarantees an even, edge-to-edge cook without overcooking the outer layers! If checking with a digital meat thermometer after removing it from the bag, it should read right around 130°F–135°F for a warm, tender center.
Q: Can I make this more medium-rare or medium instead of rare?
A: Yes! While lean tenderloin is at its absolute best rare to medium-rare, you can easily adjust the doneness:
–Rare / Tender Medium-Rare (Recommended): Water temperature at 140°F (60°C) for 45 minutes.
–Medium-Rare to Medium: Raise the water temperature slightly to 145°F–150°F (63°C–65°C) and hold for 45 to 60 minutes
–Quick Skillet Method: Alternatively, slice the cooked beef into medallions and give them a quick 15-second flash sear in a hot buttered skillet right before serving.
Chef’s Notes for Rice Cooker Sous-Vide Success
- Temper the Meat (Crucial Step): Let the salted beef rest at room temperature for 1 to 2 hours. Cold meat placed in a warm water bath will drop the water temperature and cook unevenly.
- Precise 1% Salt Ratio: Weighing your salt to 1% of the meat’s total weight (8g salt for 800g / ~1.3–1.4 lbs of beef) ensures perfectly balanced seasoning from edge to center.
- Protect the Bag: Always place a small heat-safe bowl or silicone trivet at the bottom of your rice cooker pot so the vacuum bag does not touch the direct heating element.
Perfect for Meal Prep & Next-Day Dishes
While this tenderloin steak makes a stunning centerpiece for a main dinner course, it is also incredible for weekly meal prep!
I normally store any leftover slices directly in a Ziploc bag—either with or without the extra koji sauce—and keep them in the refrigerator (or freeze them for later). The gentle cooking method keeps the meat tender and juicy even when served chilled. Tossing these cold or room-temperature slices over a vibrant grain bowl, tucked into a sourdough sandwich, or served over a crisp steak salad makes for an effortless, high-protein lunch the next day.
